A clutch mechanism plays roles to transfer rotational driving force which is inputted via a rotational driving force input system (hereinafter referred to as “input system”) to a rotational driving force output system (hereinafter referred to as “output system”) and to discontinue the transfer. The clutch mechanism has friction plates which are connected to the input system and clutch plates which are connected to the output system and brought into pressure contact with the friction plates. Accordingly, engagement and disengagement between the input system and the output system are effected by bringing the friction plates and the clutch plates into pressure contact with each other and by releasing the friction plates and the clutch plates from the pressure contact, respectively.
The pressure contact between the friction plates and the clutch plates is effected by pressing the friction plates against the clutch plates, and the pressing of the friction plates is effected by means of a clutch pressing mechanism. The clutch pressing mechanism has a pressure plate which directly presses the friction plates, an elastic member which supplies the pressing force to the pressure plate, and a retainer which applies a load to a spring portion of the elastic member.
The spring load of the elastic member is determined as follows. First, from a maximum torque value of a motor vehicle such as a two-wheeled motor vehicle, a necessary clutch load which is required for preventing slide from occurring between the friction plates and the clutch plates is determined. Then, the spring load of the elastic member is so determined that the load with which the pressure plate presses the friction plates is greater than the necessary clutch load.
As described above, the spring load of the elastic member is determined on the basis of the maximum torque value and set to be constant irrespective of an engine speed. It is thereby possible to prevent slide between the friction plates and the clutch plates over a whole range of the engine speed.
On the other hand, however, since the spring load of the elastic member is determined on the basis of the maximum torque value, the spring load is inevitably large in a case of a motor vehicle in which engine torque is high.
In particular, in a case of a two-wheeled motor vehicle, a pressure plate with which a clutch pressing mechanism is provided and which directly presses friction plates is operated by a clutch lever handled by a driver. In other words, by driver's grasping of the clutch lever, the pressure plate is moved against a spring load of an elastic member, and the friction plates are released from the pressing applied thereto by the pressure plate. The pressure contact between clutch plates and the friction plates is thereby removed to effect discontinuation of transfer of rotational driving force inputted via an input system to an output system, i.e., so-called “disengagement of clutch”. Thus, when “disengagement of clutch” is effected, the driver is required to grasp a clutch lever with a force capable of overcoming the spring load of the elastic member to operate the clutch lever.
Accordingly, in driving of a two-wheeled motor vehicle with a high torque engine, a driver is likely to be required to apply a large force to the clutch lever to effect manual operation thereof. This puts a heavy burden on the driver.
Actually, in driving of a two-wheeled motor vehicle with a high torque engine for a long time, or in driving along a street with a high traffic density which requires gear shiftings repeatedly, driver's grasping power is likely to be susceptible to lowering. To tackle such situations, enhancement of physical strengths such as grasping power and the like and high driving skill which enables smooth gear shifting are required. Under these circumstances, those who can enjoy driving of a two-wheeled motor vehicle have been a limited number of people. Further, in a case of women, they often have soft grips. Accordingly, due to difficulty in clutch lever operation itself, types of motor vehicles of which driving they can enjoy have tended to be restricted.
As a method for reducing a force required for operating a clutch lever, there has been a method in which amount of movement of a pressure plate is reduced as compared with that in a conventional clutch mechanism. By this method, however, it has not been easy to realize accuracy in removing a pressing force applied by the pressure plate to friction plates and ensuring a condition where pressure contact between the friction plates and the clutch plate is surely removed.
Further, in Japanese Unexamined Patent Publication No. Hei8-61389 (No. 61389/1996), as a structure for reducing a clutch pedal treadling force in a clutch pressing assembly having a diaphragm spring as an elastic member, there is disclosed such a structure that when a clutch is engaged, a pressing force of a pressing member is multiplied by a lever mechanism and the greater pressing force is exerted on a pressure plate, and by adjusting a lever ratio of a lever member of the lever mechanism, a pressing force of a diaphragm spring is set to be at a low level to reduce a clutch pedal treadling force.
In this clutch pressing assembly, however, only an elastic member in the form of a diaphragm spring which provides a constant load is used. Accordingly, if the constant spring load is set to be small, the load with which the pressure plate presses friction plates is reduced over a whole range of engine speed. Therefore, pressure contact between the friction plates and clutch plates is not necessarily ensured over a whole range of engine speed. For example, when the engine speed is high, the pressure contact is not ensured.
As described above, it has not necessarily been easy to concurrently satisfy the demand to reduce a force required for operating a clutch lever and the demand to increase a force with which a clutch pressing mechanism presses friction plates, and further, to ensure pressure contact between the friction plates and clutch plates over a whole range of engine speed.
Under such circumstances, it is an object of the present invention to provide a clutch disc pressing assembly which is capable of maintaining a force with which a clutch pressing mechanism presses friction plates to ensure pressure contact between the friction plates and clutch plates over a whole range of engine speed, and yet which enables many people to enjoy driving of a motor vehicle irrespective of their physical strengths.